Keywords

Abstract

Exclusive use of male test subjects is common practice in animal research, based in part on a long-held assumption that females are more variable than males because of the estrus cycle. Recent studies examining a variety of traits in rodents found no evidence of different variability between the sexes for most outcomes; for traits in which one sex was more variable, males were more variable slightly more often than females

(Prendergast et al. 2014, Becker et al. 2016). These studies leave unresolved whether the sources of variability in males and females are similar. For instance, similar overall variability could arise from high variability across days in females and high variability between male individuals. Alternatively, males and females may have similar variability across days, even for traits that vary across the estrous cycle.

In the present study, we compared variability of male and female Syrian hamsters (Mesocricetus auratus) across multiple tests. Female Syrian hamsters spontaneously ovulate on a 4-day cycle, allowing exact identification of cycle phase. Ovariectomized female hamsters were compared to intact (or sham operated) females and males. Subjects were habituated to tasks and tested over two estrous cycles (or the equivalent duration) for behavior in open field, novel object recognition, and sucrose preference tests. When investigating estrous cycling, ovariectomized females and males were arbitrarily assigned an estrous cycle, with “faux estrous phases” assigned for each day. The distribution of faux estrous phases was assigned to match the distribution of estrous phase days for true cycling intact females.

Groups tracked across days of testing overlapped in mean response and variability in most testing measures, excluding distance traveled in the open field test. When mapping testing days over estrus phases, variation between groups was mostly similar across groups. In cases when variability fluctuated, it often fluctuated most on day 3 of estrous in intact females, when the coefficient of variation tended to decrease or increase compared to variation on other days. However, significant fluctuations occurred across faux estrous cycle days in some male and ovariectomized female tests, suggesting that a larger sample size is needed to definitively claim whether the estrous cycle was controlling variability on this day, and not random variation.